The detection and measurement of short, transient pulse modulated signals emitted from an acoustic signal source such as, for example, an active sonar source has been the subject of recent efforts in the array signal processing field. However, much of this effort has not addressed three important practical issues: (1) how low the signal to noise ratio is (less than −10 dB); (2) the existence of self-noise transient signals; and (3) the presence of strong tonals. Therefore, prior art signal models and detection methods have not performed satisfactorily when applied to detecting, for example, actual underwater acoustic signals.
In an actual underwater environment, machinery installed on surface ships and submarines, for example, inevitably generates a variety of harmonic resonance signals. These signals are called tonals. Tonals detected by an acoustic receiver can be much stronger than the emitted signal. Due to various underwater biological effects and flow induced resonances, self-noise transient signals can also interfere with the performance of the acoustic receiver.
Experimental data indicates that self-noise transient signals appear very similar to the emitted signal in the time domain. Some known characteristics of self-noise transient signals include: (1) their arrival time can be modelled using Poisson distribution; (2) their frequency is randomly distributed; and (3) their duration varies from a few milliseconds to a few hundred milliseconds.
Existence of self-noise transient signals is a major factor which contributes to the degradation of the false alarm rate performance of an underwater acoustic receiver. The duration of a pulse modulated emitted signal can be as short as a few milliseconds and as long as one second. Multipath delays also interfere with the emitted signal. Since it is desirable to provide a long range detection capability, the received emitted signal is often weak compared to environmental (noise) signals since the signal to noise ratio is low. In addition, the received emitted signal is corrupted by background “pink” noise as will be understood by those skilled in the art.
In order to detect the emitted signal and to measure its characteristics, it is necessary to enhance the aforenoted signal to noise ratio so a the reconstituted signal can be reliably recognized and measured. Since apriori knowledge of the emitted signal is not available, conventional match filter techniques will not aid in the enhancement of the signal to noise ratio. A number of developed detection algorithms have been proposed based on a statistical hypothesis test method. Unfortunately, the statistical models of self-noise transient signals are not known. Therefore, such methods are not useful.
The object of the present invention is to provide a method for detecting emitted signals which enhances the signal to noise ratio of the signals in an actual noisy environment, and which method is amenable to real time implementation.